Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support to the next decade. The ability to provide high bit rates is a key measure for LTE.
The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is to be finalized as Release 8 (LTE). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. The detailed system requirements are given in. In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP), and its affinity to different transmission bandwidth arrangements. Single-Carrier Frequency Division Multiple Access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmission power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques, and a highly efficient control signaling structure is achieved in LTE (Release 8).
LTE Architecture
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN consists of eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
Component Carrier Structure in LTE (Release 8)
The downlink component carrier of a 3GPP LTE (Release 8) is subdivided in the time-frequency domain in so-called sub-frames. In 3GPP LTE (Release 8) each sub-frame is divided into two downlink slots, wherein the first downlink slot comprises the control channel region (PDCCH region) within the first OFDM symbols. Each sub-frame consists of a give number of OFDM symbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release 8)), wherein each of OFDM symbol spans over the entire bandwidth of the component carrier. The OFDM symbols are thus each consists of a number of modulation symbols transmitted on respective NRBDL×NscRB subcarriers as also shown in FIG. 4.
Assuming a multi-carrier communication system, e.g. employing OFDM, as for example discussed in the “Long Term Evolution” work item of 3GPP, the smallest unit of resources that can be assigned by the scheduler is one “resource block”. A physical resource block is defined as NsymbDL consecutive OFDM symbols in the time domain and NscRB consecutive subcarriers in the frequency domain as exemplified in FIG. 4. In 3GPP LTE (Release 8), a physical resource block thus consists of NsymbDL×NscRB resource elements, corresponding to one slot in the time domain and 180 kHz in the frequency domain (for further details on the downlink resource grid, see 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, version 8.7.0, section 6.2, available at http://www.3gpp.org and incorporated herein by reference).
General Structure for Downlink Physical Channels
The general downlink LTE baseband signal processing according to 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation”, version 8.6.0, March 2009, section 6.3 (available at http://www.3gpp.org and incorporated herein by reference) is exemplarily shown in FIG. 6. Further details on the LTE downlink can be found in 3GPP TS 36.211, section 6. A block of coded bits is first scrambled. Up to two code words can be transmitted in one sub-frame.
In general, scrambling of coded bits helps to ensure that receiver-side decoding can fully utilize the processing gain provided by channel code. For each codeword, by applying different scrambling sequence for neighboring cells, the interfering signals are randomized, ensuring full utilization of the processing gain provided by the channel code. The scrambled bits are transformed to a block of complex modulation symbols using the data modulator for each codeword. The set of modulation schemes supported by LTE downlink includes QPSK, 16-QAM and 64-QAM corresponding to 2, 4 or 6 bits per modulation symbol.
Layer mapping and precoding are related to MIMO applications. The complex-valued modulation symbols for each of the code words to be transmitted are mapped onto one or several layers. LTE supports up to four transmit antennas. The antenna mapping can be configured in different ways to provide multi antenna schemes including transmit diversity, beam forming, and spatial multiplexing. Further the resource block mapper maps the symbols to be transmitted on each antenna to the resource elements on the set of resource blocks assigned by the scheduler for transmission. The selection of resource blocks depends on the channel quality information.
Downlink control signaling is carried out by three physical channels:                PCFICH to indicate the number of OFDM symbols used for control signaling in a sub-frame (i.e. the size of the control channel region)        PHICH which carries downlink ACK/NACK associated with UL data transmission        PDCCH which carries downlink scheduling assignments and uplink scheduling assignments.Downlink Reception in 3GPP LIE        
In 3GPP LTE (Release 8), where there is only once component carrier in uplink and downlink, the PORCH is sent at a known position within the control signaling region of a downlink sub-frame using a known modulation and coding scheme. As the determination of the downlink resources assigned to the user equipment depends on the size of the control signaling region of the sub-frame, i.e. the number of OFDM symbols used for control signaling in the given sub-frame, the user equipments needs to decode the PCFICH in order to obtain the signaled PORCH value, i.e. the actual number of OFDM symbols used for control signaling in the sub-frame.
If the user equipment is unable to decode the PCFICH or obtains an erroneous PCFICH value, this PCFICH detection error will result in the user equipment not being able to correctly decode the L1/L2 control signaling (PDCCHs) comprised in the control signaling region, so that all resource assignments contained therein are lost.
Physical Downlink Control Channel (PDCCH) Assignment
The physical downlink control channel (PDCCH) carries scheduling grants for allocating resources for downlink or uplink data transmission. Each scheduling grant is defined based on Control Channel Elements (CCEs). Each CCE corresponds to a set of Resource Elements (RES). In 3GPP LTE, one CCE consists of 9 Resource Element Groups (REGs), where one REG consists of four REs.
The PDCCH is transmitted on the first one to three OFDM symbols within a sub-frame. For a downlink grant on the physical downlink shared channel (PDSCH), the PDCCH assigns a PDSCH resource for (user) data within the same sub-frame. The PDCCH control channel region within a sub-frame consists of a set of CCE where the total number of CCEs in the control region of sub-frame is distributed throughout time and frequency control resource. Multiple CCEs can be combined to effectively reduce the coding rate of the control channel. CCEs are combined in a predetermined manner using a tree structure to achieve different coding rate.
In 3GPP LTE, PDCCH can aggregate 1, 2, 4 or 8 CCEs available for control channel assignment is a function of several factors, including carrier bandwidth, number of transmit antennas, number of OFDM symbols used for control and the CCE size, etc. Multiple PDCCHs can be transmitted in a sub-frame.
On a transport channel level, the information transmitted via the PDCCH is also refereed L1/L2 control signaling. L1/L2 control signaling is transmitted in the downlink for each user equipment (UE). The control signaling is commonly multiplexed with the downlink (user) data in a sub-frame (assuming that the user allocation can change from sub-frame to sub-frame). Generally, it should be noted that user allocation might also be performed on a TTI (Transmission Time Interval) basis where the TTI length (in the time domain) is equivalent to either one or multiple sub-frames. The TTI length may be fixed in a service area for all users, may be different for different users, or may even by dynamic for each user. Generally, then the L1/L2 control signaling needs only be transmitted once per TTI.
Furthermore, the PDCCH information sent on the L1/L2 control signaling may be separated into the Shared Control Information (SCI) and Dedicated Control Information (DCI)—please note that sometimes the acronym DCI is also referred to as Downlink Control Information.
DCI transports downlink or uplink scheduling information, or uplink power control commands for one RNTI (Radio Network Terminal Identifier). The RNTI is a unique identifier commonly used in LTE for destining data or information to a specific user equipment. The RNTI is implicitly included in the DCI by masking the CRC of the encoded payload data of the DCI with the RNTI. On the user equipment side, if decoding of the payload size of data is successful, the user equipment detects the DCI to be destined to the user equipment by checking whether the CRC on the decoded payload data using the “unmasked” CRC (i.e. after removing the masking using the RNTI) is successful. Please note that the masking of the CRC code is performed by scrambling the CRC with the RNTI.
In 3GPP LTE (Release 8) the following different DCI formats are defined:
Uplink DCI Formats                Format 0 used for transmission of UL SCH assignments        Format 3 is used for transmission of TPC commands for PUCCH and PUSCH with 2 bit power adjustments (multiple UEs are addressed)        Format 3A is used for transmission of TPC commands for PUCCH and PUSCH with single bit power adjustments (multiple UEs are addressed)        
Downlink DCI Formats                Format 1 used for transmission of DL SCH assignments for SIMO operation        Format 1A used for compact transmission of DL SCH assignments for SIMO operation        Format 1B used to support dosed loop single rank transmission with possibly contiguous resource allocation        Format 1C is for downlink transmission of paging, RACH response and dynamic BCCH scheduling        Format 1D is used for compact scheduling of one PDSCH codeword with precoding and power offset information        Format 2 is used for transmission of DL-SCH assignments for closed-loop MIMO operation        Format 2A is used for transmission of DL-SCH assignments for open-loop MIMO operation        
For further information on the LTE physical channel structure in downlink and the PDSCH and PDCCH format, see Stefania Sesia et al., “LTE—The UMTS Long Term Evolution”, Wiley & Sons Ltd., ISBN 978-0-47069716-0, April 2009, sections 6 and 9.
Blind Decoding of PDCCHs at the User Equipment
In 3GPP LTE (Release 8), the user equipment attempts to detect the DCI within the PDCCH using so-called “blind decoding”. This means that there is no associated control signalling that would indicate the CCE aggregation size or modulation and coding scheme for the PDCCHs signaled in the downlink, but the user equipment tests for all possible combinations of CCE aggregation sizes and modulation and coding schemes, and confirms that successful decoding of a PDCCH based on the RNTI as described above. To further limit complexity a common and dedicated search space in the control signaling region of the LTE component carrier is defined in which the user equipment searches for PDCCHs.
In 3GPP LTE (Release 8) the PDCCH payload size is detected in one blind decoding attempt. The user equipment attempts to decode two different payload sizes for any configured transmission mode, as highlighted in Table 1 below. Table 1 shows that payload size X of DCI formats 0, 1A, 3, and 3A is identical irrespective of the transmission mode configuration. The payload size of the other DCI format depends on the transmission mode.
TABLE 1DCI formatspayload sizetransmissionpayload size Xdifferent from Xmode0/1A/3/3A1Cbroadcast/unicast/paging/powercontrol1Mode 1DL TX modes1Mode 22AMode 32Mode 41BMode 51DMode 61Mode 71Mode 1SPS-Modes1Mode 22AMode 32Mode 41Mode 7
Accordingly, the user equipment can check in a first blind decoding attempt the payload size of the DCI. Furthermore, the user equipment is further configured to only search for a given subset of the DCI formats in order to avoid too high processing demands.
Uplink Power Control in 3GPP LTE
Uplink power control controls the transmit power of the different uplink physical channels. The setting of the user equipment's transmit power PPUSCH (measured in dB) for the physical uplink shared channel (PUSCH) transmission in subframe i is defined by the equation:PPUSCH(i)=min{PCMAX,10 log10(MPUSCH(i))+PO—PUSCH(j)+α(j)·PL+ΔTF(i)+f(i)}
Similarly, for physical uplink control channel (PUCCH) transmission the transmit power control is given by,PPUCCH(i)=min{PCMAX,P0—PUCCH+PL+h(nCQI,nHARQ)+ΔF—PUCCH(F)+g(i)}where δPUSCH (/δPUCCH) is a UE specific correction value, also referred to as a Transmit Power Control (TPC) command and is included in PDCCH with DCI format 0 (/downlink DCI formats) or jointly coded with other TPC commands in PDCCH with DCI format 3/3A whose CRC parity bits are scrambled with TPC-PUSCH (/PUCCH)-RNTI. The current PUSCH (/PUCCH)-power control adjustment state is given by f(i) which is defined by:
            f      ⁡              (        i        )              =                  f        ⁡                  (                      i            -            1                    )                    +                                    δ            PUSCH                    ⁡                      (                          i              -                              K                PUSCH                                      )                          ⁢                                  ⁢        for        ⁢                                  ⁢        PUSCH              ,          ⁢            g      ⁡              (        i        )              =                  g        ⁡                  (                      i            -            1                    )                    +                        ∑                      m            =            0                                M            -            1                          ⁢                                  ⁢                              δ            PUCCH                    ⁡                      (                          i              -                              k                m                                      )                                ,      for    ⁢                  ⁢    PUCCH  if accumulation is enabled based on the UE-specific parameter Accumulation-enabled provided by higher layers.DCI Format 3/3A in 3GPP LTEDCI Format 3
DCI format 3 is used for the transmission of TPC commands for PUCCH and PUSCH and includes transmit power control fields (TPC fields) that have a size of two bits, i.e. allow for 2-bit power adjustments. The DCI format 3 consists of a given number of N TPC fields, wherein each TPC field includes a TPC command for a different UE. A UE is only assigned one TPC field in a PDCCH of DCI format 3. If multiple PDCCHs of DCI format 3 are transmitted in the sub-frame, still there is only one TPC field in one PDCCH assigned to the UE. The number of TPC fields in the DCI format 3 is given by
  N  =      ⌊                  L                  format          ⁢                                          ⁢          0                    2        ⌋  and where, Lformat 0 is equal to the payload size of format 0 (scheduling of an uplink assignment) before CRC attachment, including any padding bits appended to format 0. The parameter tpc-index provided by higher layers determines the index to the TPC field/command for a given user equipment. If
            ⌊                        L                      format            ⁢                                                  ⁢            0                          2            ⌋        <                  L                  format          ⁢                                          ⁢          0                    2        ,a bit of value zero is be appended to format 3. Multiple PDCCH with DCI of format 3 transmitted within a sub-frame are addressed to groups of UEs by masking the CRC with respective RNTIs, so-called TPC-RNTIs that are specifically defined in the system for power control signaling. Each user equipment is assigned one of the TPC-RNTIs which is used in the blind detection of the PDCCH carrying the DCI of format 3.
The Table 2 below shows the mapping of TPC command values in DCI format 3 for PUSCH as specified in 3GPP LTE (Release 8)—see 3GPP TS 36.212 “Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding”, version 8.8.0, and 3GPP TS 36.213, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures”, version 8.8.0, available at http://www.3gpp.org.
TABLE 2TPC Command FieldAccumulatedAbsolute δPUSCH [dB]in DCI format 0/3δPUSCH [dB](only for DCI format 0)0−1−410−1211334Similarly TPC values are also defined for PUCCH.Format 3A
DCI format 3A is used for the transmission of TPC commands for PUCCH and PUSCH with single bit power adjustments. With respect to its structure SCI format 3A includes twice as many TPC fields as DCI format 3, but the TPC field size is ½ of the TPC field size for DCI format 3, i.e. only one bit. DCI format 3A thus consists of a given number of M=Lformat 0 TPC fields, wherein each TPC field includes a TPC command for a different UE, where Lformat 0 is again equal to the payload size of format 0 before CRC attachment, including any padding bits appended to format 0. Further, also for DCI format 3A higher layer signaling provides the UE with the parameter tpc-index provided by higher layers determines the index to the TPC field/command for a given user equipment and the TPC-RNTI to use for blind detection.
Table 3 below shows the mapping of TPC command values in DCI format 3A for both PUCCH and PUSCH as specified in 3GPP LTE (Release 8).
TABLE 3TPC Command Field inDCI format 3AδPUSCH [dB]0−111Further Advancements for LTE—LTE-Advanced (3GPP LTE-A)
The frequency spectrum for IMT-Advanced was decided at the World Radio communication Conference 2007 (WRC-07) in November 2008. Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved which is also referred to as “Release 10”. The study item covers technology components to be considered for the evolution of E-UTRA, e.g. to fulfill the requirements on IMT-Advanced. Two major technology components which are currently under consideration for LTE-A are described in the following.
In order to extend the overall system bandwidth, LTE-A (Release 10) uses carrier aggregation, where two or more component carriers as defined for LTE (Release 8)—see FIG. 3 and FIG. 4 discussed above are aggregated in order to support wider transmission bandwidths e.g. up to 100 MHz and for spectrum aggregation. It is commonly assumed that a single component carrier does not exceed a bandwidth of 20 MHz.
A terminal may simultaneously receive and/or transmit on one or multiple component carriers depending on its capabilities:                An LTE-Advanced (Release 10) compatible mobile terminal with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple component carriers. There is one Transport Block (in absence of spatial multiplexing) and one HARQ entity per component carrier.        An LTE (Release 8) compatible mobile terminal can receive and transmit on a single component carrier only, provided that the structure of the component carrier follows the Release 8 specifications.        
It is also envisioned to configure all component carriers LTE (Release 8)-compatible, at least when the aggregated numbers of component carriers in the uplink and the downlink are same. Consideration of non-backward-compatible configurations of LTE-A (Release 10) component carriers is not precluded. Accordingly, it will be possible to configure a user equipment to aggregate a different number of component carriers of possibly different bandwidths in the uplink and the downlink.
PDCCH Structure and Cross-Carrier Scheduling 3GPP LTE-A
As indicated above, in 3GPP LTE-A (Release 10) carrier aggregation, i.e. the use of multiple component carriers in uplink and downlink, respectively will be used. It is presently envisioned by the 3GPP to utilize cross-carrier scheduling, which means that a (single) PDCCH on one of the downlink component carriers can assign downlink (Physical Downlink Shared Channel—PDSCH) or uplink resources (on the Physical Uplink Shared Channel—PUSCH) on multiple component carriers (see 3GPP Tdoc. R1-094959, “TP for TR36.814 on downlink control signaling for carrier aggregation”, agreed in the 3GPP RAN 1 meeting no. 58, available at http://www.3gpp.org and incorporated herein by reference), Motivations for the use of cross-carrier scheduling are heterogeneous network operation, support extension carrier operation, efficient scheduling in case of PDCCH CCE blocking probability, etc.
It has been agreed in the 3GPP that the PDCCH on a (downlink) component carrier can assign PDSCH resources on the same component carrier and PUSCH resources on a single linked UL component carrier. Rel-8 PDCCH structure (same coding, same CCE-based resource mapping) and DCI formats are used on each component carrier. Furthermore, the PDCCH on a component carrier can be used to assign PDSCH or PUSCH resources in one of multiple component carriers using the carrier indicator field (CIF), where 3GPP LTE (Release 8) DCI formats are extended with a fixed 3 bits carrier indicator field, and 3GPP LTE (Release 8) PDCCH structure (same coding, same CCE-based resource mapping) is reused. The presence of carrier indicator field may be semi-statically configured.
With respect to transmit power control, the mechanisms proposed for 3GPP LTE (Release 8) may need some adaption and optimization for use in 3GPP LTE-A (Release 10). In contrast to 3GPP LTE (Release 8), carrier aggregation of multiple component carriers in uplink and downlink will be used in 3GPP LTE-A (Release 10), which requires enhanced signaling mechanisms for power control. As will be apparent from the following, a simple reuse of 3GPP LTE (Release 8) transmit power control. i.e. performing it simply for each uplink component carrier, is not efficient in terms of asymmetric uplink/downlink component carrier aggregation, its enhanced demands on blind decoding attempts by the user equipments and the related use of processing resources and (battery) power, control signaling overhead, etc.